System and method for pH control of lean MEG product from MEG regeneration and reclamation packages

ABSTRACT

A MEG stream having a first pH level is contacted with a CO 2 -rich gas stream to yield a MEG product having a second different and lower pH level. The system and method can be readily incorporated into a slipstream MEG recovery package, with a source of the MEG stream being a MEG regeneration section of the package. The CO 2 -rich gas could be a vented CO 2  stream from the MEG reclamation section of the package. Unlike hydrochloric and acetic acid overdosing, CO 2  overdosing of the lean MEG stream does not lead to rapid acidification of the MEG product to be stored or injected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application which claims priority toU.S. patent application Ser. No. 14/973,443, filed Dec. 17, 2015, whichwas a divisional application that claimed priority to U.S. patentapplication Ser. No. 14/500,295 filed on Sep. 29, 2014, U.S. Pat. No.9,216,934, all of which are incorporated herein by reference.

BACKGROUND

Slipstream MEG recovery packages use a regeneration section to removewater from an incoming rich MEG feed stream and produce a lean MEGstream. A portion of this lean MEG stream is routed to a reclamationunit or section where the salt component is removed to yield asalt-free, pH neutral, lean MEG stream. This salt-free lean MEG streamis then blended with the remaining lean MEG stream to produce a lean MEGproduct having up to 3 wt % dissolved salts and available forre-injection into the gas production line as hydrate inhibitor.

For gas fields where significant quantities of calcium and otherdivalent cations are present in the formation water, a calcium removalunit or section is located upstream of the regeneration section. Thecalcium is removed from the rich MEG stream by elevating the pH throughthe addition of sodium or potassium carbonates, hydroxides, or somecombination thereof. The lean MEG exits the calcium removal section withan elevated pH, typically above 9.5.

Because carbonate and hydroxide are often added in excess of therequired stoichiometric quantity, un-reacted carbonate and hydroxide iscarried through the regeneration system and into the lean MEG product.Removal of water from the rich MEG in the regeneration section furtherelevates the pH of the lean MEG product sent for reinjection. Mixingthis high pH lean MEG with the calcium-rich formation water in the gasproduction pipeline can lead to increased scaling of the pipeline byprecipitation of, for example, calcium carbonate.

Therefore, a need exists to reduce the pH of the lean MEG product priorto injection and, in turn, mitigate pipeline scaling. Acidification ofthe lean MEG with hydrochloric acid (HCl) is an option but overdosingwith hydrochloric acid can lead to rapid reduction in pH to levels atwhich corrosion of carbon steel pipework and vessels may occur.

SUMMARY

A lean MEG stream having a first pH level (e.g., pH>9.5) is contactedwith a CO₂-rich gas stream to yield a lean MEG product having a seconddifferent pH level, which may be in a range of 6.5 to 7.0. The CO₂-richgas could be a vented CO₂ stream from a MEG reclamation unit.

Carbon dioxide has advantages to hydrochloric acid (HCl) and acetic acid(CH₃CO₂H) for pH control because overdosing with CO₂—i.e., adding it inexcess of the required stoichiometric quantity—does not lead to thereduction in pH observed with hydrochloric acid or the accumulation ofacetates observed with acetic acid.

Embodiments of this disclosure may reduce the pH of lean MEG productprior to injection, mitigate the potential for pipeline scaling, reduceor eliminate use of dosing with organic or inorganic acids to controlthe pH of the lean MEG product, may be less sensitive to overdosingconditions, and does not cause rapid reduction in pH levels when anoverdose condition occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features can be understoodin detail, a more particular description may be had by reference toembodiments, some of which are illustrated in the appended drawings,wherein like reference numerals denote like elements. It is to be noted,however, that the appended drawings illustrate various embodiments andare therefore not to be considered limiting of its scope, and may admitto other equally effective embodiments.

FIG. 1 is a schematic of an embodiment of a system and method of thisdisclosure. A vessel located downstream of a MEG regeneration sectionreceives a high pH lean MEG stream and allows the steam to come intocontact with a CO₂-rich gas.

FIG. 2 is a graph illustrating a lean MEG stream with alkalinity presentas sodium carbonate as the stream is treated with CO₂, acetic acid, andhydrochloric acid.

FIG. 3 is a graph illustrating a lean MEG stream with alkalinity presentas sodium hydroxide as the stream is treated with CO₂, acetic acid, andhydrochloric acid.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS

10 Vessel

15 Rich MEG steam (untreated stream)

20 Lean MEG stream (treated stream)

21 Portion of lean MEG stream 20

30 MEG regeneration unit or section

CO₂-rich gas 40

Lean MEG product exiting 10

60 MEG reclamation unit or section

61 Salt-free lean MEG stream

70 Calcium removal unit or section

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of some embodiments of the present disclosure. However,it will be understood by those of ordinary skill in the art that thesystem and/or methodology may be practiced without these details andthat numerous variations or modifications from the described embodimentsmay be possible.

In the specification and appended claims: the terms “connect”,“connection”, “connected”, “in connection with”, and “connecting” areused to mean “in direct connection with” or “in connection with via oneor more elements”; and the term “set” is used to mean “one element” or“more than one element”. Further, the terms “couple”, “coupling”,“coupled”, “coupled together”, and “coupled with” are used to mean“directly coupled together” or “coupled together via one or moreelements”. As used herein, the terms “up” and “down”, “upper” and“lower”, “upwardly” and downwardly”, “upstream” and “downstream”;“above” and “below”; and other like terms indicating relative positionsabove or below a given point or element are used in this description tomore clearly describe some embodiments of the disclosure.

Referring to FIG. 1, an embodiment of a system and method for adjustinga pH level of a lean MEG (treated) steam includes a vessel 10 whichreceives a lean MEG stream 20 from a lean MEG source such as aregeneration unit or section 30 of a slipstream MEG recovery package.Typically, stream 20 has a pH level above 9.5, as does rich MEG(untreated) stream 15 upstream of the regeneration section 30. Withinvessel 10, this high pH lean MEG stream 20 comes into contact with aCO₂-rich gas 40 (i.e., greater than 50% CO₂ content). Vessel 10 can be acontactor vessel of a kind known in the art.

The CO₂ in gas 40 forms acidic solutions when dissolved in the MEG-watermixture of stream 20, thereby reducing the pH. A lean MEG product 50having a second lower pH exits the vessel 10. In embodiments, product 50may have a pH level of 6.5 to 7. No inorganic acids such as HCl ororganic acids such as acetic or citric acid are used for reducing the pHto this level.

The CO₂-rich gas 40 can be from any source but, in some embodiments, isa vent stream from a reclamation unit or section 60 of the slipstreamMEG recovery package. Similar to MEG regeneration section 30, MEGreclamation section 60 is of a kind well-known in the art.

A salt-free lean MEG stream 61 which exits the reclamation section 60can be mixed with the lean MEG stream 20 prior to stream 20 enteringvessel 10. Additionally, a portion 21 of the lean MEG stream 20 whichexits the regeneration section 30 can be routed to the reclamation unit60.

In slipstream MEG recovery packages that make use of a calcium removalunit or section 70 upstream of the regeneration unit 30, excesscarbonate that finds its way into the reclamation section 60 degrades toform CO₂ (and hydroxide) under the elevated temperature, low pressureregime of a flash separator (not shown).

Referring to FIGS. 2 and 3, unlike hydrochloric and acetic acidoverdosing, CO₂ overdosing within vessel 10 does not lead to rapidacidification of the lean MEG product 50. In a CO₂ overdosing condition,the pH level remains above 6 whereas in an acetic acid and hydrochloricacid overdosing condition the pH level falls below 4 and 2 respectively.Therefore, the system and method of this disclosure is less sensitive tooverdosing conditions than prior art methods.

As mentioned above, acidification with CO₂ removes the risk which occurswith inorganic acids (HCl) and the absence of carboxylates (acetate),namely, overdosing to the point of potentially damaging pH levels. Inaddition, carboxylates are highly soluble in MEG and are difficult toremove once added to the MEG system. The accumulation of carboxylatescan lead to operational problems as the density and viscosity of the MEGincreases with increasing carboxylate content. Hydrochloric acidconverts readily to salt plus water; carbon dioxide converts tobicarbonate which is much more easily managed in the MEG system thancarboxylates. Although the CO₂ reduces the pH, the ‘alkalinity’ (OH—plus HCO₃— plus CO₂) is not reduced.

To examine the “scaling” potential for the system and method, thefollowing software simulation was run employing OLI Analyzer v 9.1.5(OLI Systems, Inc., Cedar Knolls, N.J.).

Starting solution: 90 wt % MEG (on salt-free basis) at 40° C. containing30,000 mg/kg_(solvent) sodium chloride, 250 mg/kg_(solvent) of sodiumcarbonate and 25 mg/kg_(solvent) of sodium hydroxide. The pH of thismixture was 10.053 or about 10 (see Table 1, col. A, below).

Acidification: The MEG solution was neutralized to pH=7.0 and to pH=6.5using HCl acetic acid and CO₂. Quantities of HCl, CH₃CO₂H and CO₂ addedare shown in Table 1, rows 12-14, below.

Scaling Test: Scaling potential of the acidified solutions wasdetermined by adding in separate simulations MgCl₂, CaCl₂, FeCl₂, SrCl₂and BaCl₂ to the lean MEG solutions at the quantities shown in Table 1,rows 19-23.

TABLE 1 Software Simulation of Scaling Potential. 1 A B C D E F G 2 3TEMP 40 40 40 40 40 40 40 4 5 H2O g 100,000 100,000 100,000 100,000100,000 100,000 100,000 6 MEG g 900,000 900,000 900,000 900,000 900,000900,000 900,000 7 NaCl g 30,000 30,000 30,000 30,000 30,000 30,00030,000 8 Na2CO3 g 250 250 250 250 250 250 250 9 NaOH g 25 25 25 25 25 2525 10 11 ACIDIFICATION 12 HCl g 0 136 — — 160 — — 13 CHCO2H g 0 — 228 —— 279 — 14 CO₂ g 0 — — 239 — — 478 15 16 pH — 10.05 7.01 7.01 7.01 6.506.50 6.50 17 18 SCALING TEST POST ACIDIFICATION 19 MgCl2 for Mgprecipitation g as Mg(OH)2 20 CaCl2 for Ca precipitation g 1.4 840 880250 4300 5300 800 as CaCO3 21 FeCl2 for Fe precipitation g 0.1 1.5 1.70.8 6.7 7.7 2.4 as FeCO3 22 SrCl2 for Sr precipitation g 0.5 650 660 2003900 3950 620 as SrCO3 23 BaCl2 for Ba precipitation g 0.2 7 13.7 2.1 3580 6.3 as BaCO3

Results: For the starting solution (col. A, pH=10.0) precipitation ofdivalent cations as carbonate occurs on addition of 1.4 g of CaCl₂.After acidification to pH 7.0 with HCl, the quantity of calcium chlorideadded before precipitation of CaCO₃ increases to 840 g from 1.4 g. Theeffect with acetic acid is similar with precipitation starting at 880 gof CaCl₂. The equivalent scaling point with carbon dioxide occurs at 250g, less than that for HCl or acetic acid but a considerable improvementon the 1.4 g for the untreated sample.

Similar trends are observed for the other divalent cations (Fe, Sr, Ba)although some are more insoluble than others. Iron, in particular, tendsto precipitate out readily. At pH=6.5 (col. E-G) the trends agree withthose shown at pH=7.0 (col. B-D), i.e. precipitation of divalent cations(Ca, Fe, Sr, and Ba) from the lean MEG is inhibited by addition of CO₂to the alkaline lean MEG mixture.

Although the preceding description has been described herein withreference to particular means, materials and embodiments, it is notintended to be limited to the particulars disclosed herein; rather, itextends to all functionally equivalent structures, methods, and uses,such as are within the scope of the appended claims.

What is claimed:
 1. A MEG recovery vessel including: an inlet in fluidcommunication with a MEG stream exiting a MEG treatment, the MEG streamhaving a first pH level; a port in fluid communication with a gasstream, the gas stream containing more than 50% CO₂; an outlet for theMEG stream after contact with the gas stream; and a vent for the gasstream after contact with the MEG stream; wherein the MEG stream aftercontact with the gas stream has a second pH level lower than the firstpH level; and wherein the gas stream after contact with the MEG streamhas a lowered CO₂ content.
 2. A MEG recovery vessel according to claim 1wherein an amount of CO₂ in the gas stream is at least equal to astoichiometric quantity effective for achieving a reduced pH level ofthe MEG stream.
 3. A MEG recovery vessel according to claim 1 wherein anamount of CO₂ in the gas stream is greater than a stoichiometricquantity effective for achieving a reduced pH level of the MEG stream.4. A MEG recovery system according to claim 1 wherein the second lowerpH level is at least
 6. 5. A MEG recovery vessel according to claim 1wherein the second lower pH level is
 7. 6. A MEG recovery vesselaccording to claim 1 wherein the MEG stream is mixed with a second MEGstream having a different pH level than the first pH level.
 7. A MEGrecovery vessel according to claim 6 wherein the different pH level is7.
 8. A MEG recovery vessel according to claim 1 wherein the MEG streamis mixed with a second MEG stream having a different salt content thanthat of the MEG stream.
 9. A MEG recovery vessel according to claim 1wherein the MEG treatment includes a MEG regeneration unit.
 10. A MEGrecovery vessel according to claim 1 wherein the gas stream is from aMEG reclamation unit.
 11. A MEG recovery vessel according to claim 1wherein the MEG treatment lowers a non-MEG content of a mixturecontaining the MEG stream.
 12. A MEG recovery gas/liquid contactorvessel comprising: an inlet in fluid communication with a MEG streamexiting a MEG treatment; a port in fluid communication with a gas streamcontaining more than 50% CO₂; an outlet for a reduced pH MEG stream; anda different outlet to vent a reduced CO₂ gas stream; wherein the MEGstream and the gas stream come into contact with one another within thegas/liquid contactor vessel.
 13. A method for pH control of a MEGstream, the method comprising: contacting a MEG stream with a gasstream, the MEG stream being from a MEG treatment, the gas streamcontaining more than 50% CO₂.
 14. A method according to claim 13 whereinan amount of CO₂ in the gas stream is at least equal to a stoichiometricquantity effective for achieving a reduced pH level of the MEG stream.15. A method according to claim 13 wherein an amount of CO₂ in the gasstream is a stoichiometric quantity effective for achieving a reduced pHlevel of the MEG stream.
 16. A method according to claim 13 furthercomprising mixing the MEG stream with a second MEG stream having adifferent pH level than that of the MEG stream.
 17. A method accordingto claim 13 further comprising mixing the MEG stream with a second MEGstream having a different salt content than that of the MEG stream. 18.A method according to claim 13 wherein a pH level the MEG stream is atleast 6 after contact with the gas stream.
 19. A method according toclaim 13 wherein the MEG treatment includes a MEG regeneration unit. 20.A method according to claim 13 wherein the gas stream is from a MEGreclamation unit.
 21. A method according to claim 13 wherein the MEGtreatment lowers a non-MEG content of a mixture containing the MEGstream.